1. Field of the Invention
The present invention relates to an apparatus for photoelectrically detecting an optical image. More particularly, the present invention relates to a measuring apparatus for photoelectrically detecting an optical image of an object to be measured and detecting a focusing state of an imaging optical system or a distance to the object to be measured.
2. Description of the Prior Art
Conventional focus detection apparatus for single lens reflex cameras are roughly classified into relative displacement detection systems and sharpness detection systems. In a relative displacement detection system, two images of substantially the same portion of an object to be photographed are formed on a pair of photoelectric element array, in each of which a number of photoelectric elements are arrayed, by light rays passed through different regions of a pupil of a phototaking lens. Photoelectric outputs from the photoelectric element arrays are used for operations to detect a relative displacement of the optical images on the photoelectric element arrays. Focus detection is performed based on the detection result obtained. On the other hand, in a sharpness detection system, images of an object to be photographed formed by a phototaking lens are directed onto pair of photoelectric element arrays which are arranged before and after a predetermined focal plane of the phototaking lens. Photoelectric outputs from the photoelectric element arrays are used for performing operations to detect sharpness of the image of the object and to thereby perform focus detection. The operations performed on the photoelectric outputs involves the spatial frequency components in a given spatial frequency band of the image of the object. Low spatial frequency components remain in the image of the object, that is, in the photoelectric electric outputs, even if the phototaking lens is considerably spaced apart from the in-focus position and the object image is blurred accordingly. Therefore, using only the low-order spatial frequency components for the operations, a front-focus or a rear-focus (a front-focus or a rear-focus state indicates a state wherein an object image is formed in front of or behind the predetermined focal plane) can be discriminated even in the case wherein a defocus amount (the defocus amount is the amount of displacement between the predetermined focal plane of an imaging optical system and an object image along the direction of the optical axis) is large. However, focus detection using low spatial frequency components is adversely affected by various electrical or optical errors, and the in-focus position itself cannot be determined with high accuracy. On the other hand, if the spatial frequency components to be subjected to operations are relatively high, the detection result is less adversely affected by such errors and a correct defocus amount can be obtained if the object image is located near the predetermined focal plane. That is, high-precision focus detection can be performed. However, when the phototaking lens is considerably spaced apart from the in-focus position, high spatial frequency components of an object image decreases significantly. Then, precision of focus detection is significantly lowered, and discrimination between a front-focus and a rear-focus cannot even be performed in some cases. The relative displacement detection system has another disadvantage in that if the operations are performed using only high spatial frequency components, an in-focus signal (to be referred to as a pseudo in-focus signal hereinafter) may be generated even if the phototaking lens is at some position distant from the true in-focus position. If the operations are performed using photoelectric outputs in which high spatial frequency components are not separated from low spatial frequency components, the disadvantages involved in operations using either one of the high- and low-order spatial frequency components are increased and the advantages obtained in operations using the other of the high and low spatial frequency components are impaired.
More particularly, focus detection is roughly classified into (1) correct calculation of a defocus amount when a phototaking lens is near a in-focus position, and (2) approximate calculation of a defocus amount or discrimination between a front-focus and a rear-focus when the phototaking lens is significantly spaced apart from the in-focus position. Ideal focus detection can be performed using mainly high spatial frequency components for the former (1) and using low spatial frequency components for the latter (2). This also applies to a sharpness detection system. There are cases wherein an object itself contains a large amount of high spatial frequency components and only a small amount of low spatial frequency components, and vice versa. It has been impossible to perform high-precision detection for all of such various types of objects.